Methods and apparatus for minimally invasive modular interbody fusion devices

ABSTRACT

The invention is a modular interbody fusion device for fusing adjacent spinal vertebrae that is adapted to be implanted in a prepared interbody space including a first modular segment having a width including a first rail extending at least partially along one side of the width and beyond a periphery of a body portion of the first modular segment, a second modular segment having a width and slidably connected to the first rail on one side of the width and having a second rail extending at least partially along another side of the width and beyond a periphery of a body portion of the second modular segment, a third modular segment having a width and slidably connected to the second rail on one side of the width and wherein the device has an expanded position in which the second and third modular segments are extended along the first and second rails and positioned in a generally end to end configuration spaced apart by the rails prior to implantation and an implanted position in which the modular segments are positioned in a generally side by side configuration that defines a unitary body that mimics the planar shape of the vertebra such that the device contacts and supports the adjacent vertebra.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 11/974,185 filed Oct. 11, 2007; which claims the benefit of U.S. Provisional Application No. 60/860,329 filed Nov. 21, 2006, each if which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to an implantable orthopedic fusion device for fusing joints in a patient such as a vertebral interbody fusion device. More particularly, the present invention relates to a rail-based modular interbody fusion device of predetermined size and shape.

BACKGROUND OF THE INVENTION

Joint fusion or arthrodesis is a common approach to alleviate the pain due to deteriorated and/or arthritic joints. Joint fusion involves inducing bone growth between two otherwise mobile bones in a joint, which alleviates pain by immobilizing and stabilizing the joint. The joint is generally fused in its most functional position. The ankle, wrist, finger, toe, knee and vertebral joints are all examples of joints that may be fused to alleviate pain associated with unstable, deteriorated joints.

The spinal motion segment consists of two adjacent vertebral bodies, the interposed intervertebral disc, as well as the attached ligaments, muscles, bony processes and the facet joints. The disc consists of the end plates at the surfaces of the vertebral bones, the soft inner core, called the nucleus pulposus and the annulus fibrosus ligament that circumferentially surrounds the nucleus and connects the vertebrae together. In normal discs, the nucleus cushions applied loads, thus protecting the other elements of the spinal motion segment. The nucleus in a normal disc responds to compression forces by bulging outward against the vertebral end plates and the annulus fibrosus. The annulus consists of collagen fibers and a smaller amount of elastic fibers, both of which are effective in resisting tension forces. However, the annulus on its own is not very effective in withstanding compression and shear forces.

As people age the intervertebral discs often degenerate naturally. Degeneration of the intervertebral discs may also occur in people as a result of degenerative disc disease. Degenerative disc disease of the spine is one of the most common conditions causing back pain and disability in our population. When a disc degenerates, the nucleus dehydrates. When a nucleus dehydrates, its ability to act as a cushion is reduced. Because the dehydrated nucleus is no longer able to bear loads, the loads are transferred to the annulus and to the facet joints. The annulus and facet joints are not capable of withstanding their increased share of the applied compression and torsional loads, and as such, they gradually deteriorate. As the annulus and facet joints deteriorate, many other effects ensue, including the narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints lose their structural stability and subtle but pathologic motions occur between the spinal bones.

As the annulus loses stability it tends to bulge outward and may develop a tear allowing nucleus material to extrude. Breakdown products of the disc, including macroscopic debris, microscopic particles, and noxious biochemical substances build up. The particles and debris may produce sciatica and the noxious biochemical substances can irritate sensitive nerve endings in and around the disc and produce low back pain. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the trunk are attempted.

Degeneration of a disc is irreversible. In some cases, the body will eventually stiffen the joints of the motion segment, effectively re-stabilizing the discs. Even in the cases where re-stabilization occurs, the process can take many years and patients often continue to experience disabling pain. Extended painful episodes of longer than three months often leads patients to seek a surgical solution for their pain.

Several methods have been devised to attempt to stabilize the spinal motion segment. Some of these methods include: applying rigid or semi-rigid support members on the sides of the motion segment; removing and replacing the entire disc with an articulating artificial device; removing and replacing the nucleus; and spinal fusion involving permanently fusing the vertebrae adjacent the affected disc.

Spinal fusion is generally regarded as an effective surgical treatment to alleviate back pain due to degeneration of a disc. The fusion process requires that the vertebral endplates be prepared by scraping the surface of the existing vertebral bone to promote bleeding and release of bone growth factors, and placing additional bone or suitable bone substitute onto the prepared surface. Devices of an appropriate size made from rigid materials such as metals (including titanium and tantalum), some plastics (including polyetheretherketone (PEEK), or carbon fiber-filled PEEK), and allograft bone (primarily from donor femurs) are commonly inserted into the prepared disc cavity as part of the interbody fusion procedure to help distract and stabilize the disc space and put the vertebra into proper position while the bone growth process takes place. The interbody fusion procedure may be accomplished from an anterior, transforaminal, or a posterior surgical approach.

Most devices used in interbody spinal fusion require a relatively large opening that is typically larger than the dimensions of the rigid and unitary fusion device or cage that is to be inserted, examples of such devices include, U.S. Pat. No. 5,026,373 to Ray et al., U.S. Pat. No. 5,458,638 to Kuslich et al., and the NOVEL™ PEEK Spacers from Alphatec. In fact, many methods of interbody fusion, for example the method and device described in U.S. Pat. No. 5,192,327 to Brantigan, require bilateral placement of unitary devices through fairly large surgical openings. As with any surgical procedure, the larger the surgical access required, the higher the risk of infection and trauma to the surrounding anatomy.

There exists minimally invasive spinal fusion devices such as is disclosed in U.S. Pat. No. 5,549,679 to Kuslich and U.S. Pat. No. 6,997,929 to Manzi et al. The device disclosed in the U.S. Pat. No. 5,549,679 is a porous mesh bag that is filled in situ. The U.S. Pat. No. 6,997,929 is directed to a series of wafers that are vertically stacked to distract and support the vertebral endplates. U.S. Pat. No. 5,702,454 to Baumgartner discloses plastic beads which may be inserted one at a time into an intervertebral space on a flexible string. Further, U.S. Pat. No. 5,192,326 to Bao discloses hydrogel beads encased in a semi-permeable membrane.

While such minimally invasive technologies permit smaller access incision through the annulus (i.e. an annulotomy) to be used in a fusion procedure, the resulting fusion devices do not have the mechanical and dimensional features of the more rigid unitary fusion devices used in traditional surgical approaches and are less able to distract and stabilize the disc space. Thus, there is a need for a minimally invasive spinal fusion implant that could better emulate the mechanical and structural characteristics of a rigid unitary fusion device.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for a rail-based modular interbody fusion device having a predetermined size and shape when assembled in situ. In one embodiment, the modular interbody fusion device comprises generally solid modular segments with rails that operably connect adjacent modular segments. This configuration allows the interbody spacer to be adapted for implantation via a small access incision or annulotomy through various surgical approaches, including a posterior or a lateral approach. In one embodiment, the rails operate with a sliding mechanism to connect and interlock adjacent modular segments. A stem portion of the rails that extends beyond the periphery of the body of the prosthesis is removable after implantation such that the modular segments combine to form a single device with a relatively smooth outer circumference when assembled in situ. The modular fusion device can be configured to provide full contact with and closely mimic the geometry of the surfaces of the joint being fused so as to more closely mimic the functionality of the largest existing rigid and unitary fusion devices.

In one embodiment, an interbody modular fusion device is adapted to be implanted in a prepared intervertebral space and includes at least three modular segments each having a width. The first modular segment has a first rail extending at least partially along one side of the width and beyond a periphery of the first modular segment. The second modular segment is slidably connected to the first rail on one side of the width and has a second rail extending at least partially along another side of the width and beyond a periphery of the second modular segment. The third modular segment is slidably connected to the second rail on one side of the width. The interbody fusion device has an expanded position in which the modular segments are extended along the first and second rails and positioned in a generally end to end configuration spaced apart by the rails prior to implantation. The interbody fusion device also has an implanted position in which the modular segments are positioned in a generally side by side configuration that defines a single assembled body having a generally continuous periphery that generally corresponds to the inner boundary of the annulus.

In one embodiment, each modular segment has a compressive modulus in the superior to inferior direction from about 0.5-15 GPa, such that the compressive modulus of the interbody fusion device generally corresponds to the compressive modulus of the surrounding cortical bone.

In one embodiment, locking features are provided to ensure that the modular interbody spacer is a unitary device both before and after insertion. To prevent the device from being separated prior to insertion, locking features may be provided on the rigid rails to prevent modular segments from being slid back off of the rails. This ensures that each modular segment is connected in its proper position and in the proper order. In addition, locking features may be provided on the modular segments to lock them together upon insertion. This prevents individual segments from dislocating from the assembled prosthesis and migrating outside of the annulus. Further, the interbody fusion device may include grooves, ridges, or other structures on its outer surface to contact surrounding bone and prevent the device from migrating out beyond the anterior limit of the intervertebral space.

Another aspect of the present invention comprises a method for implanting an interbody spacer. Because the modular interbody spacer may be implanted one segment at a time, a hole made in the annulus for implantation of the prosthesis may be a fraction of the size of the device in its final assembled form. The first modular segment is inserted into the intervertebral space through the small hole in the annulus. The second modular segment is then slid up the first rigid rail and into the intervertebral space until the second modular segment interlocks with the first modular segment. The tail stem of the first rigid rail is then severed from the device. This severing may be accomplished by simply snapping the rail off the device. Alternatively, the tail stem may be attached to the device by a screw, a bayonet mechanism, a twist lock or the like. As such, the rails may be removed from the device by unscrewing, or releasing the bayonet, etc. Subsequent modular segments are slid up the adjoining rigid rail into the interbody space and then interlocked with the previously inserted modular segment in a similar manner. Once all of the modular segments have been inserted and all of the tail stems severed, the modular interbody spacer is fully inserted into the patient's interbody space.

Another aspect of the present invention provides an insertion tool that may be used to aid in the insertion, positioning, and rail removal of the modular interbody spacer. The proximal end of the tool has a handle with an enclosed ratchet or roller mechanism attached to and in line with the inner lumen of an elongated tube at the distal end of the tool through which a rail may be inserted. The elongated tube may have a slit or other openings along the length of the tube to aid in threading the rails into the tube. The insertion tool may be provided with a cutting mechanism for removing the rails from the modular segments once they are fully inserted.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 is a top view of a modular interbody spacer according to an embodiment of the present invention in its inserted configuration.

FIG. 2 is a perspective view of a modular interbody spacer according to an embodiment of the present invention at a first stage of insertion.

FIG. 3 is a perspective a view of a modular interbody spacer according to an embodiment of the present invention at a second stage of insertion.

FIG. 4 is a perspective view of a modular interbody spacer according to an embodiment of the present invention at a final state of insertion.

FIG. 5 is a perspective view of an alternate embodiment of the device.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there can be seen a top view of a modular interbody spacer 100 according to an embodiment of the present invention as configured once inserted into the body. In this embodiment, modular disc prosthesis 100 comprises first 102, second 104, third 106, and fourth 108 modular segments. Interbody spacer 100 may be comprised of any suitable biomaterial, for example, a polymer, such as PEEK, a metal, such as titanium, trabecular metal, bone, or a resorbable material that may act as a scaffold for new bone growth and/or a carrier for stem cells.

Modular segments 102, 104, 106 and 108 may be inserted via a small annulotomy from a posterior or lateral approach. Interbody spacer 100 may then be constructed within the interbody space by first inserting modular segment 102 into the interbody space, then sliding modular segments 104, 106 and 108 along a series of rails wherein each segment locks with the previous segment to create an interbody spacer 100 having a final, assembled surface area that fully contacts and supports the vertebral end plates.

Interbody spacer 100 may include locking barbs that prevent individual units from backing out or extending beyond the anterior limit of the spacer. Spacer 100 may further include grooves, ridges 142 or other structures to engage the surrounding bone or otherwise prevent spacer 100 from backing out of the intervertebral space.

In a preferred embodiment, interbody spacer 100 may be made of PEEK having holes therethrough, allowing for tissue ingrowth thus promoting bony fusion. The holes 140 may be of varying size and shape. Holes 140 may be spaced apart on spacer 100 in any manner such that the compressive modulus of spacer 100 generally corresponds to the compressive modulus of the adjacent bone. Spacer 100 may also be of varying thicknesses to achieve the desired support and/or fusion of a particular intervertebral space, such as a lordotic configuration for L5-S1 fusion.

In an embodiment, prior to insertion, holes 140 of interbody spacer 100 may be packed or filled for example with, autologous bone graft, calcified or decalcified bone derivative, bone graft substitute, such as hydroxyapatite, agents to promote bone growth, such as bone morphogenetic protein, or osteogenic protein-1, antibiotics, anti-cancer agents, stem cells, biologically active cytokines, cytokine inhibitors, fibroblast growth factors, other osteoinductive and/or osteoconductive materials or any other material and combination thereof to promote fusion and/or stabilize the spinal motion segment.

In another embodiment, interbody spacer 100 may include surface modifications to provide for elution of medicants. Such medicants may include analgesics, antibiotics, anti-inflammatories, anticoagulants, antineoplastics or bioosteologics such as bone growth agents. In an alternative embodiment, spacer 100 may be comprised of a material, such as for example, porous PEEK, from which an imbibed medicant can elute. In yet another embodiment, an inner portion of the spacer 100 may be comprised of one material, while the outer portion is comprised of another material. For example, the inner portion may be comprised of a solid PEEK, while the outer portion is comprised of a porous PEEK. The surface of the porous PEEK may be coated with a bioactive agent or medicant. Spacer 100 may be imbedded with a radiopaque material, such as tantalum or titanium beads to allow for x-ray visualization of the implant.

In another embodiment, the rails may be used as fill tubes such that fill material may be injected or otherwise inserted into holes 140. Spacer 100 may also be manufactured to include channels or ducts into which fill material may be inserted via the rails.

Referring to FIG. 2, there can be seen a portion of a modular interbody spacer 100 according to the preferred embodiment of the present invention prior to insertion into the intervertebral space. In alternate embodiments, the modular interbody spacer may comprise greater or fewer numbers of modular segments and rails.

Prior to insertion, modular interbody spacer 100 further includes first 110, second 112, and third 114 rails. First modular segment 102 is rigidly attached to first rail 110 at first segment interlocking portion 116. As shown in FIG. 3, second modular segment 104 is slidably attached to first segment interlocking portion 116 at first slot 128 and rigidly attached to second rail 112 at second segment interlocking portion 118. As shown in FIG. 4, third modular segment 106 is slidably attached to second interlocking portion 118 at second slot 130 and rigidly attached to third rail 114 at third segment interlocking portion 120. Fourth modular segment 108 is slidably attached to third rail 114 at fourth slot 133.

As shown in FIG. 2, each rail 110, 112 and 114 includes a stem portion that extends beyond a periphery of the body of the spacer 100, respectively. Preferably these stem portions are long enough to permit access into the intervertebral space such that one modular segment can be positioned inside the intervertebral space while the next modular segment on the rail is still outside of the body. In an exemplary embodiment, the length of the stem portions ranges between 6 cm-20 cm. Each rail 110, 112 and 114 may further include a retaining portion to keep the device from being separated prior to insertion. The retaining portions are configured to prevent the corresponding modular segments from sliding off the rails. The retaining portions may be molded into the rails or may be separate pieces or deformations of the rails added during the manufacture of the device. Rails 110, 112, 114 may be sequentially removed from the implant as modular segments 102, 104, 106, and 108 are connected within the intervertebral space and moved laterally.

The preferred embodiment is an interbody spacer that is packaged, sterile, and ready for implantation at the surgical site. The package may include any number of modular segments. In a preferred embodiment, the package would include 5 individual modular segments. Single module packages may also be used so that the surgeon may use as many segments as desired. Since the device is fully preformed and delivered as a unitary implant, the device is under direct surgeon control until the interbody spacer is completely formed. This unitary design reduces the need for the surgeon to determine how to configure the spacer to allow for the most efficacious placement of the spacer in the intervertebral space and assures that the components' order of insertion and connection are properly achieved. The size and shape of the modular interbody spacer provides a final, assembled surface area that fully contacts and supports the vertebral end plates, stabilizing the spinal unit. In this regard, it will be understood that the modular interbody spacer 100 of the present invention may be provided in a variety of different final assembled sizes to correspond to different sizes of different intervertebral spaces.

In an alternative embodiment as shown in FIG. 5, separate guide rods 150 and a guide mechanism 152 may be used to assist in inserting and aligning the modular segments. Rod 150 may be attached to the proximal end of each modular segment. Rod 150 may be used to insert a first modular segment into position. A second guide rod may be attached to a second modular segment and used to place the second modular segment in position to mate and interlock with the first modular segment. The first rod could then be detached. Subsequent segments could be inserted by repeating the process.

In an embodiment, a modular segment may include a tapped hole 154 such that rod 150 may be screwed into hole 154. Rod 150 does not participate in the interlocking mechanism of modular segments. In an embodiment, rod 150 may either be made of the same material as the modular segments, or rod 150 may be comprised of a different material, including, but not limited to, plastics such as PEEK, or metals such as stainless steel or titanium. According to one aspect of the present invention, rod 150 may be integral to the modular segments. For example, rod 150 may be injection molded from a plastic or machined from a plastic or metal.

In another embodiment of the present invention, rod 150 may be formed separately from the modular segments and then joined to the modular segments via a mechanical method such as a mating thread, twist-lock, snap-lock or such, or by the use of adhesives or other material joining methods such as thermal and ultrasonic welding. One advantage to using a mechanical method of joining rod 150 to the modular segments is the potential to re-engage the modular segments for removal from the disc space, should the need arise. The removal sequence of rods 150 from the modular segments following implantation of the modular segments in the disc space is the same as for interlocking rails.

In an embodiment, modular interbody spacer 100 may be introduced through an access tube that is inserted partially into the intervertebral space. The access tube is at least 3 inches long and preferably about 6 inches long. It should be noted that although the insertion of modular intervertebral spacer 100 is described in relation to a four-segment embodiment, embodiments having any other number of segments would be inserted in a similar fashion.

During insertion, slots 128, 130, 133 slide along the stem portions of rails 110, 112, 114 and onto segment interlocking portions 116, 118, 120. Slots 128, 130, 133 and segment interlocking portions 116, 118, 120 may be provided with locking features to prevent separation of modular segments 102, 104, 106 and 108. Locking features, such as a barb or stud or a series of barbs or studs, may be provided such that once a slot is slid onto a segment interlocking portion, it cannot be slid back off of it. A ratchet and pawl may also be used to lock modular segments together. A ratchet release tool may also be provided in case separation of modular segments is desired once they are locked together.

Various modifications to the disclosed apparatuses and methods may be apparent to one of skill in the art upon reading this disclosure. The above is not contemplated to limit the scope of the present invention, which is limited only by the claims below. 

1-31. (canceled)
 32. A modular interbody fusion device for fusing adjacent spinal vertebrae that is adapted to be implanted in a prepared interbody space, the device comprising: a first modular segment having a first width including a first rail extending at least partially along one side of the first width and beyond a first periphery of a first body portion of the first modular segment; a second modular segment having a second width and slidably connected to the first rail on one side of the second width and having a second rail extending at least partially along another side of the second width and beyond a second periphery of a second body portion of the second modular segment; a third modular segment having a third width and slidably connected to the second rail on one side of the third width; and an insertion tool for sliding the second modular segment along the first rail and the third modular segment along the second rail, wherein the device has an expanded position in which the second and third modular segments are extended along the first and second rails and positioned in a generally end to end configuration spaced apart by the rails prior to implantation and an implanted position in which the modular segments are positioned in a generally side by side configuration that defines a unitary body that mimics the planar shape of the vertebra such that the device contacts and supports the adjacent vertebra, and wherein the insertion tool includes a separation mechanism to remove the first rail and the second rail following assembly of the unitary body. 